1. Field of the Invention
The present invention relates to an electrolyte comprising a high molecular weight compound and a battery using the electrolyte.
2. Description of the Related Art
In recent years, a large number of portable electronic devices such as camera/VTR (video tape recorder) combination systems, cellular phones or portable computers have been introduced, and a reduction in size and weight of the devices has been required. Moreover, in order to be able to carry the portable electronic devices for a longer time, the volume energy density of the portable electronic devices has been improved so as to increase the capacity thereof Accordingly, as portable power sources for the devices, research and development of batteries, specifically lithium-ion secondary batteries which have a thinner profile or can be folded have been actively promoted. As an electrolyte of such a battery with a high degree of flexibility in shape, for example, a gel electrolyte in which a high molecular weight compound holds an electrolyte solution, or a solid-high molecular weight electrolyte in which an electrolyte salt is dispersed in a high molecular weight compound is cited, and research on them has been actively conducted.
As the high molecular weight compound used in the solid high molecular weight electrolyte, silicon gel, acryl gel, acrylonitrile gel, a modified polyphosphazene polymer, polyethylene oxide, polypropylene oxide, a conjugated polymer thereof, a cross-linked polymer thereof, a modified polymer and the like have been reported. However, these high molecular weight compounds have insufficient film strength and poor adhesion to an electrode. The adhesion between the electrolyte and the electrode has a large influence on battery performance. For example, when the adhesion between them is poor, contact resistance between the electrolyte and the electrode increases, thereby the internal resistance in a battery increases. Moreover, it is difficult to transfer ions between the electrolyte and the electrode in an ideal state, so the battery capacity is reduced. In addition, when the battery is used in this state for a long time, a charge/discharge current becomes nonuniform, and a lithium dendrite is easily deposited. As a result, the charge/discharge cycle life is reduced. Therefore, a critical problem that the battery does not meet a fundamental and important requirement for commercialization, that is, “the battery can be stably used for a long time” arises. Therefore, it is extremely important to bond the solid high molecular weight electrolyte or a gel electrolyte to the electrode with sufficient adhesion strength.
In order to overcome such a problem, a method of cross-linking by a reaction between trifunctional polyethylene glycol and a diisocyanate derivative (refer to Japanese Unexamined Patent Application Publication No. Sho 62-48716), and a method of cross-linking by a polymerization reaction of polyethylene glycol diacrylate (refer to Japanese Unexamined Patent Application Publication No. Sho 62-285954) have been attempted. Moreover, a technique of using a fluorine-based polymer with high molecular weight as a high molecular weight compound has been proposed (refer to Japanese Unexamined Patent Application Publication No. Hei 11-312536).
However, in the methods described in Japanese Unexamined Patent Application Publication Nos. Sho 62-48716 and Sho 62-285954, even if the film strength can be improved, an unreacted substance or a solvent used for a reaction remains, thereby it is difficult to sufficiently secure adhesion to the electrode. Moreover, a drying step for removing the solvent is absolutely necessary, thereby productivity is low. Therefore, a further improvement is required. An electrolyte described in Japanese Unexamined Patent Application Publication No. Hei 11-312536 has high viscosity, so in terms of obtaining higher capacity, when the thickness of an active material layer increases to improve efficiency of filling an active material into a battery, the electrolyte less easily permeates into the neighborhood of a current collector, so it is difficult that the electrolyte comes into contact with the active material layer in the neighborhood of the current collector in an ideal state. As a result, internal resistance increases, thereby cycle characteristics, load characteristics and low temperature characteristics decline.
Therefore, by recent and rapid development in portable electronic devices, the transmission speed or the processing speed of the devices has been improved, or the devices have become multifunctional, thereby batteries are required to be used at high load current. However, these techniques become incapable of coping with the requirements under present circumstances.